This disclosure relates to generating plasmas in pulsed power systems.
Plasmas are collections of charged particles. The particles are separated by distances that are small enough to allow individual particles to influence multiple adjacent charged particles. Electrostatic interactions predominate over ordinary gas kinetics and bulk interactions with the collection predominate over boundary effects. Because of these properties, many consider plasmas to be fourth state of matter and distinct from solids, liquids, and gases.
Plasmas are used in a variety of commercially important processes. For example, in sputtering, the charged particles of a plasma collide with a target and cause the ejection of target material. The ejected target material can deposit on a substrate to form a film. By controlling the energy, duration, and other characteristics of the plasma, the characteristics of films deposited by sputtering can also be controlled. Other examples of commercially important plasma processes include other physical and chemical vapor deposition processes, plasma nitriding, plasma desmearing, plasma cleaning, plasma activation of substrates, and plasma etching. These processes are also impacted by the characteristics of the plasmas used in the processes.
The characteristics of a plasma, and the effectiveness of plasma processes, are impacted by the power delivery that charge particles and generates the plasma. For example, regions with enhanced electrical conductivity (e.g., arcs) may form in a plasma. In film deposition processes, arcs can lead to film inhomogeneities and lack of process control.
To control the characteristics of a plasma and the improve the effectiveness of plasma processing, power delivery to the plasma can be pulsed. In pulsed systems, the amount of power delivered to the plasma changes over time. For example, the delivered power can include a transient followed by a return to a steady state or a periodic variation between two states. For example, the polarity of the power can be inverted. Power can be pulsed independently of the characteristics of the plasma (e.g., according to a fixed schedule) or can be generated in response to the characteristics of the plasma. By pulsing the power delivered to charge particles, plasma arcing can be reduced or prevented. In film deposition processes, DC, pulsed DC, and AC power systems can be pulsed to hinder arc formation and increase the quality of deposited films.
In practice, the pulsing of the power used to generate a plasma is commonly achieved using a “pulsing unit.” Pulsing units receive the output of one or more power supplies and implement the changes in power delivery that result in pulsing. Pulsing units include relatively expensive, high-power components that are used for pulsing power delivery. The input powers are switched using high power switches, generally implemented using high power transistors. Pulsing units also generally include high power reactive components (e.g., inductors, capacitors) that store input power for discharge into plasmas under the control of the high power switches.